Reversible optical information-recording medium

ABSTRACT

Phase-change type, reversible optical information recording medium, being possible for recording, reproducing, erasing, and rewriting of information, by use of a laser beam. This invention, consists of recording thin film of ternary elements, for example, containing Ge, Te, Sb/or Bi or quaternary elements containing the fourth element of Se with which a part of Te is replaced, which is established on such surface—flat substrates as glass or plastics. In this case, the component ratio of Te and Se is selected not to be excess for other elements, such as Ge, Sb/or Bi so as to be fixed as stable compounds of stoichiometric compositions of GeTe, Sb 2 Te 3 /or Bi 2 Te 3 , or GeSe, Sb 2 Se 3 /Bi 2 Se 3  when crystallized. Strictly speaking, a concentration of each component is selected to have proper ratio of the number of atoms each other so as to represent whole composition as the sum of each component. By this treatment, it is possible to have high crystallization speed and long cyclability of recording/erasing. The effect of Se is to increase the viscosity of the system and to make easily to obtain amorphous state; and moreover, by selecting of proper amount diplaced with Te, it is possible to obtain the composition of recoding film superior in both characteristics of recording (amorphization) and erasing (crystallization).

This application is a Continuation of application Ser. No. 07/832,647,filed Feb. 10, 1992, now abandoned, which in turn is a Continuation ofapplication Ser. No. 07/644,419, filed Jan. 22, 1991, now abandoned,which in turn is a Continuation of application Ser. No. 07/236,311,filed Jun. 10, 1988, now abandoned, which in turn is a Continuation ofapplication Ser. No. 06/909,673, filed Sep. 22, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to phase-change type, reversible opticalinformation-recording medium, being possible for recording, reproducing,erasing, and rewriting of information, by use of a laser beam.

2. Description of the Prior Arts

It is well known that some kinds of Te-based alloy film producecomparatively easily reversible phase transition by irradiation of alaser beam. Since, among them, the composition rich in Te-componentmakes it possible to obtain an amorphous state by relatively low powerof laser, the application to recording medium has been so far tried.

For example, S. R. Ovsinsky et al. have first disclosed in U.S. Pat. No.3,530,441 that such thin films as Te₈₅Ge₁₅ and Te₈₁Ge₁₅S₂Sb₂ produce areversible phase-transition according to light with high density energysuch as a laser beam. A. W. Smith has also disclosed a film ofTe₉₂Ge₂As₅ as a typical composition, and he has clarified that it couldmake recording (amorphization) and erasing (crystallization) runs ofabout 10⁴ times, and erasing (Applied Physics Letters, 18 (1971) p.254). Moreover M. Chen et al. have disclosed that a film of Te₈₇Ge₈Sn₅as a typical composition could make repeating runs of more than 10⁶times in a static test (Applied Physics Letters, 46 (1985) p. 734).Generally, in the recording medium utilizing the phase change betweenamorphous state and crystalline state, the procedure of the transitionfrom crystal to amorphous state is used to recording process, attachingimportance to recording speed. Since the above-mentioned compositionsutilize those near eutectic mixture of Te and additives, they have lowermelting points of about 400° C. and can be easily amorphized. On theother hand, in erasing run, the procedure from amorphous state tocrystal is used; since there exist many free Te-chain-structures insideeach above-mentioned composition, progress of crystallization isobstructed by them. Therefore, as the chains at random direction remaininside even after solidification, it is necessary to keep the amorphousstate for sufficiently long time and at high temperature (near Tg) inorder to arrange the orientation, namely obtain the perfect crystallinestate. There was a problem that the composition rich in Te could not inprinciple increase the erasing speed.

The existence of free Te reduces long-term thermal stability,particularly after repeating of recording/erasing run, and moreover itbecomes a cause to restrict the cycling number itself. Though, it isassumed that in a thin film at “as deposited” state, Te atoms areuniformly dispersed with other atoms in a film, there occurs, during therepeating runs of recording/erasing, the separation between a part veryrich in Te (relatively lower melting point) and a part rich in stablecompound of Te and the additives (relatively higher melting point).Therefore, there occurs the phase separation and then the recordingcharacteristics change. In this case, the composition with extremelyabundant Te is very unstable as amorphous state with low crystallizationtemperature, and also gives a result to reduce the thermal stability.This phenomenon is assumed that it is mainly due to the fact that thecomposition near eutectic point could not exist as the stable compoundand then it would transfer to more stable state in terms of internalenergy. In the case of the extreme difference of characteristics,especially melting point, between Te and stoichiometric Te compound withadditives produced by crystallization, the phase segregation proceedsmore quickly, and it is difficult for separated phase to return again tothe original uniform state. As a result, the recording and erasing couldnot be performed. As a way to solve the problem, it is necessary toincrease the laser power, keep the film at a temperature above meltingpoints of Te and Te compound for a long time, and quench after mixingthem well again; by such treatments meaning of increasing sensitivity ofrecording by using excessive Te has come to be in vain. It has beenimpossible to realize the above treatments actually in dynamic system.Namely, in the composition excessive in Te, it could not be realized toobtain the satisfied characteristics such as the rewriting speed,thermal stability, and cyclability.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to supply a reversible opticalinformation-recording medium with the improved erasing speed withoutreduction of recording sensitivity. Also, the other object of thisinvention is to supply a reversible optical information-recording mediumwith long cyclability of recording erasing. Moreover, the other objectis to supply a reversible optical information-recording mediumunchangeable in thermal stability and with high reliability even afterrepeating runs of recording and erasing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross section showing the structure of working example forreversible optical information-recording medium of this invention.

FIGS. 2a-2 d illustrate composition of recording film applied toreversible optical information-recording medium of this invention andthe composition diagram showing a tolerance of composition.

FIG. 3 is a graph showing the phase-transition temperature of recordingfilm with main composition.

FIG. 4 is a graph showing amorphizing sensitivity of recording film withmain composition.

FIG. 5 is a graph showing a crystallization speed of recording film withmain composition.

FIG. 6 is a graph showing the change in amorphizing sensitivity andcrystallization speed in the case of addition of Se to main composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in a ˜ c in FIG. 1, an optical information-recording medium ofthis invention consists of a recording layer (4) sandwiched bydielectric thin films (2, 3) such as SiO₂, Al₂O₃, ZnS on substrate (1)with flat surface, made of plastics such as PMMA and polycarbonate,metals such as Al and Cu or glass, etc. In this invention, thedielectric layer is not always necessary, but it is effective inreducing thermal damage of plastic materials or the transformation andevaporation of the recording layer itself caused by repeatingirradiation of laser beam. It is also possible to stick a protectingplate on this layer.

This invention is characterized by a recording layer. For example, arecording layer consists of ternary elements containing Te, Ge, Sb/orBi, or quarternary elements containing the fourth element of Se withwhich a part of Te is replaced. As shown by a thick line in FIG. 2(a),the main composition is located on the line linking the compositionpoints of Ge₅₀Te₅₀ and Sb₂Te₃/or Bi₂Te₃. In this invention, the maincomposition is shown by the following formula:

 x·50Ge(Te,Se)+(1−x)·20Sb₂(Te,Se)₃ /or

(1−x)·20Bi₂(Te,Se)₃ or

(Te,Se)_(60−10x)Ge_(50x)(Sb/or Bi)_(40−40x)

(wherein 0.05≦x≦0.8)

Here, the concentration of Se is not related to the value of x, and itis 30 atomic % or less of whole, namely its maximum value is 30 atomic%. Each composition is permitted to have some range as shown in FIGS.2b-2 d as a domain.

The following is to describe the fundamental concept, the concreteconstituent elements in the recording layer, and the reason for thedecision of its concentration regarding the recording medium of thisinvention.

First, when Te is contained more excessively than necessary amount forformation of stoichiometric compounds with other additives in arecording film of Te phase-change type, it becomes a cause to restrictthe erasing speed (crystallization speed), thermal stability, andcyclability, as already mentioned. Therefore, the method withnon-excessive Te by devising concentrations of the additives should betaken.

However, Te alloy with only one kind of additive element, such as CdTe,SnTe, PbTe, InTe, Sb₂Te₃, and GeTe, is unsuitable for a recording layerby one or all of the following reasons:

1) as the melting point is too high, it could not be easily melted by ashort pulse of light of a laser beam,

2) as the crystallization temperature is too low, the stable amorphouscould not be obtained, and

3) as the crystallization rate is too high, the sufficient conditionsfor rapid cooling which are necessary for amorphous conversion could notbe easily obtained. Among them, GeTe has a stable amorphous phase and arelatively low melting point of 725° C. comparing with other systems.However, it is also difficult to amorphize (recording) underconsideration of power output (25 mW at most) by the present commerciallaser diode.

Then, the methods using plural additives have been tried. The trial isto lower the melting point by composing solid solution of pluralTe-alloys and, at the same time, to make amorphous phase stable. It isnot enough to make solid solution with low melting point, but it isnecessary to obtain the solid solution to be difficult in bringing aboutphase-separation by repeating run of recording/erasing and to havesufficiently high (not too high) crystallization rate.

In this invention, for example, by use of two kinds of additives, suchas Ge and Sb/or Bi, the good systems with relatively lower meltingpoint, stable amorphous phase, and high crystallization rate, could beobtained. Regarding crystallization as all Te is fixed as stoichiometriccompounds such as GeTe and Sb₂Te₃/or Bi₂Te₃, the crystallization rate ishigher than the case of excessive Te. However, it should be emphasizedthat there are several important points which are remarkable and unique,in this system, as follows:

As the first point, there exists an intermediate phase between phases ofboth ends; for example, between GeTe and Sb₂Te₃/or Bi₂Te₃, there areplural stoichiometric compounds, such as GeSb₄Te₇/or GeBi₄Te₇(GeTe+2Sb₂Te₃/or 2Bi₂Te₃), GeSb₂Te₄/or GeBi₂Te₄ (GeTe+Sb₂Te₃/or Bi₂Te₃),and Ge₂Sb₂Te₅/or Ge₃Bi₂Te₆ (2 GeTe+Sb₂Te₃/or 3GeTe+Bi₂Te₃). It isassumed that it is able to obtain a very high crystallization rate,because the internal energy in the crystalline state is lower in astoichiometric phase and the difference of energy level for an amorphousphase is larger.

As the second point, the above-mentioned compounds have mutually similarcrystal structures, and have very close characteristics. This factaffords the following effect: even if the composition of film would notstrictly coincide with one phase of the above-mentioned stoichiometriccompounds, characteristics are uniform throughout broad range ofcomposition as a whole, as the whole could be regarded as the mixture ofthe above-mentioned three phases. Therefore, there is a merit that thereis almost no change in the film characteristics, even if the phaseseparation has occurred by the repeating runs of recording/erasing,except the case of which the complete separation up to the compositionat both ends occurs.

As the third point, the melting points of the above-mentioned phases arelow and close to each other. The melting point in the system ofGeTe—Sb₂Te₃ is near 600° C., and in GeTe—Bi₂Te₃ near 570° C.; as theyare lower by more than 100° C. comparing with 725° C. of GeTe singlesubstance, melting by a laser beam could be performed relatively easily.There is also a merit that the phase separation mentioned at the secondpoint would be difficult to occur, as the melting points of each phaseare close to each other.

The fourth point has been found in the crystallization process. In thesystem of GeTe—Sb₂Te₃/or Bi₂Te₃, the stable crystal form is hexagonal,but in the crystallization process by laser radiation, single phase ofNaCl-type cubic system consisting of ternary elements, Ge, Te, and Sb orGe, Te and Bi, is obtained first (this is also the case with Se). It isadvantageous that the crystal form obtained is as isotropic as possible(namely, near the atomic structure of liquid phase or amorphous phase)in order to promote smoothly the crystallization. It is assumed that thediffusion distance of atom is shortened and then the annealing timenecessary for crystallization is shortened.

As the fifth point, sufficiently high crystallization temperature tokeep an amorphous state stable could be obtained through the broad rangeof composition in the system of GeTe—Sb₂Te₃/or Bi₂Te₃. As shown in FIG.3(a) and (b), it has been found that the crystallization temperaturesufficiently higher than room temperature was obtained until a closeconcentration of single phase of Sb₂Te₃/or Bi₂Te₃.

In this invention, adding to the above-mentioned important points of1-5, the following merit could be moreover obtained by the addition ofSe.

Se is added only in the form of displacement of Te, so that the balanceof the concentration between Te and Ge, Sb/or Bi is not demolished. Thefirst addition effect of Se is to increase the viscosity of the system.Added Se forms the Se-compounds, such as GeSe, GeSe₂, Sb₂Se₃/or Bi₂Se₃,when film is crystallized. As these compounds have higher bond energythan the corresponding Te-compounds of the same elements, they are notcompletely separated even at liquid phase. Therefore, the molten liquidhas the high viscosity, and it becomes easy to form amorphous phase. Inother words, the condition of rapid cooling to form an amorphous phasebecomes slightly mild. It is important that this characteristic does notshow the incontinuous change by the addition of Se. It was confirmedthat, though the crystallization rate of the system became low inverselyproportionally to viscosity, this change varied very continuouslyaccording to the concentration of Se and could be controlled. Therefore,in the system design, when it is necessary to make recording and erasingat high speed, the composition with low concentration of Se should beset emphasizing crystallization rate. On the contrary, when it isnecessary to make recording and erasing at low speed, the compositionwith high concentration of Se should be set, emphasizing the conditionof amorphous formation. As described above, it is possible to adjustminutely these by the composition. Actually, the limit ofSe-concentration is 30 atomic % at most, and there occurs the deficientcrystallization rate at higher concentration. The second addition effectof Se is to raise the crystallization temperature of the system. It isespecially effective in poor composition of GeTe—component, and it actsto raise the stability of amorphous phase (the recorded signal). On theother hand, the melting point does not so change by addition of Se. Itis assumed that this is due to the near melting point between Te- andSe-compounds of the same elements. It is very important that therecording sensitivity does not decrease by the addition of Se.

As described above, it was found that the ternary system of Te, Ge andSb/or Bi or the quarternary system in which a part of Te was displacedby Se showed superior characteristics as recording material forrewriting type optical disc.

Next, the manufacturing method of this invention is described. Arecording medium of this invention could be produced by the method, suchas vacuum deposition and sputtering. As for sputtering, it is possibleto use an alloy target designed from the desirable composition or acomplex mosaic target with the area corresponding to each compositionratio. In the case of vacuum deposition method, the co-evaporationmethod with plural sources is convenient to control the composition. Inthis case, it is important that the deposition rate from each source isperfectly independently controlled by preparing for four electron gunsand their electric sources and four sensors of film thickness (forexample, quartz oscillator). The degree of vacuum at deposition isenough at 10⁻⁴-10⁻⁷ torr.

This invention is explained in detail by the following detailedexamples.

EXAMPLE 1

Sample pieces of the recording film containing triple elements of Te, Geand Sb/or Bi with various compositions were prepared by theabove-described vacuum deposition method, and these characteristics wereinvestigated. The substrate of sample piece is a glass disk withthickness of 0.3 mm and diameter (φ) of 8 mm; the film thickness of arecording layer is about 1000 Å.

Characteristics were evaluated as follows:

i) Phase transition temperature, Tx

ii) Laser power necessary for beginning of amorphous conversion, Pa

iii) Laser irradiation time necessary for beginning of crystallization,τx Tx was defined as the temperature beginning the change in opticaltransmittance, when the as deposited sample piece was gradually heated.Increasing the temperature at the rate of 1° C./sec., the change of thetransmittance was monitored by He—Ne laser, and the transformation pointwas detected. The stability of amorphous phase could be evaluated bythis value. Pa shows a value measured radiation power necessary forbeginning of amorphization by irradiation of laser beam to the recordingfilm surface of the crystalline state. In this case, each sample wassufficiently precrystallized by 30 μs radiation of 2 mW laser power.Afterwards, the pulse width of radiation was fixed at 0.2 μs, and theradiation laser power beginning the amorphization was measured bychanging irradiation power. By this value, the sensitivity of amorphousconversion (the recording sensitivity) can be evaluated. τx is theirradiation time necessary for beginning of crystallization, in the caseof radiation at spot of a 1 μm or less in diameter on the recording filmat “as deposited” state by the light of laser diode through a lenssystem. In this case, the evaluation of τx was undertaken from twopoints of view assuming two crystallization processes. One of them isτx₁, the radiation time necessary for beginning of crystallization bythe radiation of relatively weak laser power (2 mW), assuming thecrystallization process at solid phase. The other is τx₂, the radiationtime necessary for beginning of crystallization in the case of quenchingfrom molten state by the radiation power of more than the above Pa. τx₁is the item related both with the energy necessary for beginning ofcrystallization and the rate of the crystallization. This value is verycorrelated with Tx. On the other hand, τx₂ is evaluated as the rate ofcrystallization itself; in this case, the same laser power as requiredfor amorphization.

FIG. 3 shows the behavior of Tx in the compositions on the line whichlinks the composition point of GeTe and that of Sb₂Te₃/or Bi₂Te₃ at thetriangle diagram of Ge—Te—Sb/or Bi.

From this figure, the phase transition temperature in the systems ofGeTe—Sb₂Te₃ and GeTe—Bi₂Te₃ is higher than 100° C. (sufficiently higherthan room temperature) in the case of more than 3-5% of x in theabove-mentioned formula, and it shows that the amorphous phase is verystable.

FIG. 4 shows the sensitivity of amorphization, Pa in the samecomposition as above. From this figure, Pa in the systems of GeTe—Sb₂Te₃and GeTe—Bi₂Te₃ has a tendency to increase as the proportion of GeTeincreases, and it increases very much in the case of more than 80-90% ofx proportion. Namely, if the proportion of GeTe or exist in the regionof 80% or less of x, it is shown that a high recording sensitivity couldbe obtained.

FIG. 5 shows τx₁ and τx₂, the irradiation time necessary for beginningof crystallization in the same composition as above. From this figure,it is found that τx₁ has a tendency to increase as the proportion ofGeTe increases, τx₂ has a tendency to increase as the proportion of GeTedecreases, and it does not exist in the case of less than 30% of x. Thecurves of τx₁ and τx₂cross at around 30-40% of x-concentration.Therefore, it is found that the region of 5-40% of x in the compositionratio is suitable for erasing method of solid phase and the region of40% or more of x ratio is suitable for erasing method of liquid phase.

Next, the example in which a part of Te was displaced by Se in the samecomposition on the above-mentioned line is described.

EXAMPLE 2

Selecting Ge₂₀Te₅₆Sb₂₄ as a typical composition in Example 1, a part ofTe has been displaced by Se in the form of Te_(56-x)Se_(x)Ge₂₀Sb₂₄, andit was prepared as a sample. By the same measurement as adapted inExample 1, the following results have been obtained.

First, it was confirmed that Tx increased monotonically as theconcentration of Se increased.

FIG. 6 shows the change in the sensitivity of amorphization Pa, and thecrystallization time τx₁ and τx₂ with the change in Se concentration.From this figure, it is found that Pa decreases by addition of Se, andsaturates at about 10 atomic %; τx₁ and τx₂ increases continuously andmonotonically as the concentration of Se increases. τx₁ increasesextremely over 20 atomic % of Se concentration; τx₂ increasesmonotonically until the concentration of Se is about 15 atomic %, but itdoes not exist at higher than 15 atomic % of Se concentration (namely,crystallization becomes difficult)

From these results, it was shown that it was possible to changecontinuously recording sensitivity and erasing rate by the displacementof Se with Te and the fine controlling of film characteristics is easilyundertaken according to the specification of the system. At that time,the addition concentration of Se is 30 atomic % at most, and thecrystallization rate is reduced extremely at higher concentration. Inorder to crystallize from liquid phase, the concentration of Se is 15atomic % at most. Moreover, under consideration of the recordingsensitivity, the concentration of Se is suitable at 5-15 atomic %.

Similar experiment was undertaken for the different composition ofTe—Ge—Sb, and the same tendency was obtained as addition effects of Se.In the system of Te—Ge—Bi, the similar results were obtained.

EXAMPLE 3

The same experiment as adopted in Example 1 was undertaken to thecomposition come off the above-mentioned composition-line. Fixing theconcentration of Ge for the composition on the above line, thecomposition ratio of Sb/or Bi was changed in the range of ±15 atomic %.As a result, the large difference from the composition on the line wasnot produced as to Tx and Pa, but τx showed a rather large value whenthe composition came off the line to both sides.

EXAMPLE 4

At each composition point of Examples 1, 2 and 3, the disk was prepared,and the repeating characteristics of recording/erasing wereinvestigated. An optical disk was prepared by laminating ZnS and arecording layer in the order, i.e. ZnS, a recording layer, ZnS . . . ,on the substrate of PMMA resin of 130 mm in diameter and thickness of1.2 mm with the light guidetrack, and the protecting plate using theUV-ray curing resin on the surface. The thickness of each layer wasabout 800 Å, 1000 Å, and 1600 Å in the order from the lowest layer tothe upper ones, and these were designed to raise the effect oflight-absorption in the recording layer. A laser beam was supplied fromthe side of substrate. A dynamic tester (deck) was equipped with acircular laser spot of 0.9 μm in diameter for recording and an ellipticlaser spot of 1×8 μm for erasing, and the two spots were continuouslyarranged in one optical head. The repeating life of recording/erasingwas investigated at a peripheral speed of 5 m/s. Each power of recordingand erasing was suitably selected according to the disk characteristics,and the recording was made at simple frequency mode of 2 MHz. The limitof life was defined as the number of times showing the decrease of 3 dBfrom initial C/N. The following conclusions of 1)-4) were obtained.

i) At the composition on the line, there exists a broad power conditionmaking possible repeating of more than 10⁶ times at more than 50 dB ofinitial C/N.

ii) Coming off far from the composition on the line, the noise isproduced at about 10² time repeatings.

iii) As shown by A₁, B₁, C₁ and D₁ in FIG. 2, the permissible widthcoming off the composition was +10 atomic % to Sb rich side and −10atomic % to Sb poor side in the case of the fixed Ge concentration; inthis range, there exists the power condition making possible repeatingof 10⁴ times. Similarly, in the range of +7 atomic % and −5 atomic %(A₂, B₂, C₂ and D₂), there exists the power condition making possiblerepeating of 10⁵ times. In the range of +5 atomic % and −3 atomic % (A₃,B₃, C₃ and D₃), there exists the power condition making possiblerepeating of 10⁶ times, though its range is narrower than thecomposition on the line. Above all, in the range of A₄, B₄, C₄ and D₄,it is possible to repeat recording/ erasing with a comparatively lowpower. Tab. 1 shows the composition of each point.

TABLE 1 Coordinates at each composition point each composition (Te, Ge,Sb/or Bi) A B C D 1 (38, 59, 3) (50, 2, 48) (70, 2, 28) (63, 34, 3) 2(42, 55, 3) (53, 2, 45) (64.5, 2, 33.5) (57, 40, 3) 3 (44.5, 52.5, 3) (55, 2, 43) (63, 2, 35) (54, 43, 3) 4 (46.5, 41.5, 12) (54, 6.5, 39.5)(62, 4, 34) (55, 39, 6)

iv) The Se concentration has not marked correlation with cyclability.

EXAMPLE 5

The cyclability of disk equipped with the film of ternary elementssystem, Te₅₈Ge₁₂Sb₃₀ and the film of quarternary elements system,Te₄₈Ge₁₂Sb₃₀Se₁₀ as a disk with typical composition were investigated bythe same method as adopted in Example 4.

As for the former disk, the results of the initial C/N of 53 dB and theC/N of 51 dB after 10⁵ time repeating were obtained by repeatingrecording/erasing at recording power of 8 mW and erasing power of 8 mW.

As for the latter disk, the results of the initial C/N of 54 dB and theC/N of 52 dB after 10⁵ times repeating were obtained by repeatingrecording/erasing at recording power of 7 mW and erasing power of 10 mW.

Similar experiments were undertaken with Sb instead of Bi in the abovecomposition. In the disk of the triple elements system, Te₅₈Ge₁₂Bi₃₀,the results of initial C/N of 50 db and the C/N of 48 dB after therepeating of 10⁵ times were obtained at the recording power of 7 mW andthe erasing power of 7 mW. Also, in the disk of the quadruple elementssystem, Te⁴⁸Ge₁₂Sb₃₀Se₁₀, the results of initial C/N of 52 dB and theC/N of 50 dB after repeating 10⁵ times were obtained at the recordingpower of 6 mW and the erasing power of 9 mW.

EXAMPLE 6

Using the disk of Example 5, the environmental test was undertaken. Ineach disk, the signal was recorded at the track where recording/erasingwere repeated 10⁵ times and the right-/left-hands neighboring tracks;then, C/N was measured at these tracks. C/N of each track was measuredafter two month leaving in thermohygrostat (80° C., 80% RH). As aresult, the change of C/N in each track was as negligibly small as−0.5˜−1.5 dB. By the observation with a metal microscope, the remarkablerust or crack was not found in each disk.

As described above, the optical information-recording medium with highsensitivity, superior heat-and-humidity resistance, and long repeatinglife of recording/erasing, was provided by this invention.

What is claimed is:
 1. A rewritable optical information storage mediumcomprising a substrate and an information storage film of phase changematerial producing reversible phase changes between amorphous andcrystalline state by laser irradiation, wherein said information storagefilm is a ternary stoichiometric compound selected from the groupconsisting of GeSb₄Te₇, GeSb₂Te₄ and Ge₂Sb₂Te₅ in the crystalline stateand sandwiched between dielectric thin films comprising ZnS.
 2. Arewritable optical information storage medium comprising a substrate andan information storage film of phase change material producingreversible phase changes between amorphous and crystalline state bylaser irradiation, wherein said information storage film is a mixture oftwo or more stoichiometric compounds selected from the group consistingof GeSb₄Te₇, GeSb₂Te₅, and Ge₂Sb₂Te₅ in the crystalline state andsandwiched between dielectric thin films comprising ZnS.
 3. A rewritableoptical information storage medium comprising a substrate and aninformation storage film of phase change material producing reversiblephase changes between amorphous and crystalline state by laserirradiation, wherein said information storage film is a ternarystoichiometric compound of Ge₂Sb₂Te₅ in the crystalline state andsandwiched between dielectric thin films comprising ZnS.